This invention relates generally to aircraft navigation, and more specifically, to an inertial reference unit which provides an independent, backup source of inertial data, for example, an attitude heading reference system (AHRS).
Aircraft utilize attitude sources (i.e. inertial reference units (IRUs) and attitude heading reference systems (AHRS)) which provide pitch, roll and heading information along with aircraft accelerations and angular body rates to various displays and/or systems in the aircraft in order to perform certain flight operations. Availability of this information is especially critical during certain flight operations such as landing in poor weather conditions.
Some of these sources of attitude and heading information are redundant to provide a measure of safety in case of equipment failure and provide for confirmation of correct readings. Dispatch of aircraft also is sometimes dependent upon the availability of redundant systems which provide attitude and heading information. For example, an aircraft may not be available for flight without two or more independent sources of attitude and heading information. In some areas of the world, replacement units to provide attitude and heading information are not readily available for installation in an aircraft. Therefore, it is possible that an aircraft may risk unsafe operations by resuming flight while not being equipped with the proper complement of attitude and heading sources. Alternatively, the aircraft could be stranded, waiting for a shipment of replacement attitude and heading sources if one or more of its attitude heading reference units are inoperable.
Aircraft are typically designed to house a particular suite of avionics instruments. Therefore, existing aircraft cannot easily add additional inertial reference units (IRUs) or attitude heading reference systems (AHRS) to their avionics suites due to space concerns, wiring and available input channels of the flight display, flight management and flight control systems. Reversionary modes, for example, an attitude mode in certain known IRUs are not available if sensor or other essential hardware in the IRU cause the unit to fail. In addition, such reversionary modes are also sometimes difficult for a flight crew to enable.